Materials are known to the art for optically recording digitally encoded information. Such materials store data by means of bit-by-bit recording with a laser. The stored data is retrieved by "reading" the element with the same or a different laser which measures the effect produced on the element by the recording laser. The "reading" laser is generally of lower power to prevent unwanted "writing" during the data retrieval mode.
The materials known for such optical recording elements generally fall under the classes of thin metal films, organic dyes in a polymer matrix, metal-containing polymers, bubble-forming materials and photographic silver halide film. Because it is desired in the art to have a direct-read-after-write capability, photographically developed silver halide, which possesses superior sensitivity and archival quality characteristics, is not widely used because of the necessity for processing before reading.
The thin metal film element generally comprises tellurium or a tellurium alloy such as that described in U.S. Pat. No. 4,222,071 issued Sept. 9, 1980 to Alan E. Bell and Robert A. Bartolini. Exposure to a recording laser results in a hole ablated in the tellurium layer by the laser. One advantage of the tellurium-based material is relatively high sensitivity, since low thermal conductivity of the metal will prevent the incident heat from the laser from being dissipated before the hole is formed. In addition, the power requirement for accomplishing such ablation is relatively low due to the relatively low melting point of tellurium compared to many other metals. However, tellurium is rapidly oxidized and thus lacks the long-term stability desired in the industry. Alloying tellurium with other materials and encapsulating the tellurium film in plastic enhances stability but usually lowers sensitivity.
An example of an optical recording element employing an organic dye in a polymer matrix may be found in U.S. Pat. No. 4,336,545, issued June 22, 1982 to Dennis G. Howe, Alan B. Marchant and Joseph J. Wrobel, which is directed to an element comprising a heat-deformable dye-polymer layer overlying a reflective surface. Pits in the recording surface which result from the laser exposure may be "read" by measuring a phase difference from light reflected from pitted and non-pitted areas. This type of material is referred to as a phase-shift type system.
An optical recording element containing a metal disposed in a polymeric matrix may be illustrated by U.S. Pat. No. 4,269,917 issued May 26, 1981 to Jerome Drexler and Eric W. Bouldin, which is directed to a reflective data storage medium comprising a low-melting colloid matrix on a substrate with a surface layer of non-filimentary silver particles disposed in said matrix wherein the volume concentration of the silver particles is greater at the surface of said colloid matrix than the interior of said matrix and wherein the surface has an areawise substantially uniform reflectivity to visible light. Laser writing is accomplished by burning through the reflective component of the element with a laser beam thereby creating a hole in the reflective component which may be detected by, for example, another laser which detects the reduced reflection as a result of the hole formation.
Optical recording media that employ bubble formation comprise a layer of vaporizable material enclosed in a higher melting point material. The laser writing beam passes through the higher melting point material without effect and vaporizes the underlying material producing a bubble that has a different reflectivity from the surrounding areas.
One of the original materials employed for in digital data storage elements was a layer of fine-grain silver halide. Not only was a separate development step necessary but the exposed spot or bit did not possess a sharp edge. Because the silver halide emulsion possesses a distribution of grain sensitivities and the exposing laser has a sinusoidal intensity distribution, grains of higher sensitivity at the edge of the spot would develop while grains of low sensitivity in the same area would not. This results in a diffuse edge rather than a sharp delineation between exposed and unexposed areas.
A summary of materials currently under consideration as materials for optical storage may be found in the Journal of Micrographics, January, 1982, pps. 33-37.
U.K. patent application G. B. 2,042,753 published Sept. 24, 1980 in the name of Keith Elden Whitmore, is directed to imaging element containing microvessels. The microvessels are disclosed as retaining photosensitive imaging material such as silver halide grains. It is also disclosed at page 11 starting at line 69 that photographic images can be read out electronically by scanning a photographic element with a light source and a photosensor. It is further stated that the product of the British application is particularly suited to this electronic readout since each microvessel can provide one scanning site. By employing infectious development to produce high contrast, the photographic image being scanned provides either a substantially uniform dark area or light area in each microvessel. The information, therefore, taken from the photographic element is already in a binary logic form and can be scanned electronically and reproduced using digital electronic equipment. However, it should be noted that the photographic element being scanned is exclusively a pictorial photographic image and thus the information was not recorded as digitally encoded information.
Copending Application of Edwin H. Land, Ser. No. 234,937 filed Feb. 17, 1981, common assignee, is directed to a photosensitive element comprising single effective silver halide grains in a predetermined spaced array. In one embodiment, the predetermined spaced array comprises a regular geometric pattern of depressions in a surface in which the single effective grains are retained.
Since both copending application Ser. No. 234,937 and U.K. patent application G. B. 2,042,753 employ silver halide, they possess two of the desired attributes for optical data recording material; those of archival stability and high sensitivity. However, because silver halide is employed, as described above, a development step is required before reading which does not render the material suitable for use in direct-read-after-write applications.
A novel element for optically recording digitally encoded information which is not susceptible to the deficiences of the prior art has now been found.